Spatio-Temporal Changes in Rainfall Patterns and Their Implications for Water Resources in Eleyele Dam Basin

Abstract

Abstract: This study investigates the spatio-temporal variability and long-term rainfall trends in the Eleyele Dam Basin, southwestern Nigeria, using daily ERA5/ERA5-Land datasets covering 1981–2023. Preprocessing ensured homogeneity, persistence control, and serial correlation adjustment. Trend analysis was performed using the non-parametric Mann–Kendall test and Sen’s slope estimator, with trend-free prewhitening applied to mitigate autocorrelation effects. Spatial rainfall distribution was generated through Inverse Distance Weighting (IDW), optimized via leave-one-out cross-validation. Results indicate a persistent north–south gradient, with annual totals ranging from ~1,266 mm in the north to ~1,539 mm in the south. The basin exhibits a distinct bi-peak regime, with maxima in June and September, and 86% of rainfall concentrated between April and October. Statistically significant negative trends were identified in March, April, May, and December (Sen’s slope −0.48 to −1.62 mm yr⁻¹), pointing to a delayed onset and weakened early wet-season contribution. Annual rainfall shows a significant decline (−5.26 mm yr⁻¹), and dry-season totals also decreased (−2.91 mm yr⁻¹). Coefficients of variation highlight increased interannual variability, especially in dry months, underscoring unreliability of inflows. Spatial maxima occur in the southern sub-catchment, where high rainfall intensity coincides with greater erosion and sediment delivery risks. These shifts imply reduced inflows, prolonged dry-season deficits, and heightened supply shortfalls. The findings underscore the need for adaptive reservoir rule-curve updates, catchment erosion mitigation, and climate-resilient water resource planning to safeguard Eleyele’s multipurpose role in Ibadan.